Characterisation of Fe–Cr–Al mixed oxides

Materials Chemistry and Physics 60 (1999) 168±176 Characterisation of Fe±Cr±Al mixed oxides Jose Manuel Gallardo Amoresa,*, Vicente Sanchez Escribanob, Guido Buscac a Departamento de QuõÁmica InorgaÁnica, Universidad Complutense, Ciudad Universitaria, E-28040 Madrid, Spain b Departamento de QuõÁmica InorgaÁnica, Universidad, Pãde la Merced, E-37008 Salamanca, Spain c Istituto di Chimica, FacoltaÁ di Ingegneria, UniversitaÁ, P.le J.F. Kennedy, I-16129 Genova, Italy Received 2 December 1998; received in revised form 4 March 1999; accepted 8 March 1999 Abstract Several samples of iron chromium aluminium mixed oxides with different composition have been prepared by coprecipitation at controlled pH starting from the corresponding nitrate salts and following dried at 393 K and calcination at 673 K for 3 h and 1173 K for 3 h. The powders have been characterised by XRD, FT±IR and DR UV±Vis spectrocopies, DTA±TG thermal analyses and measurements of BET surface area. It has been found alumina is soluble into a-FeCrO3 phase up to near 20%. These samples are stable at 1243 K with a relative high speci®c surface area. The g,q ! a phase transition is shifted towards higher temperatures by increasing Al content, being not detectable when a-FeCrO3 phase is the main phase. Surface chromates species are identi®ed by the different techniques used and their amount seem to depend directly on the speci®c surface area of each sample. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Solid solutions; Phase transition; Chromates; Coprecipitation; X-ray diffraction 1. Introduction Fe±Cr mixed oxides are widely used as catalysts for the high-temperature water gas shift reaction [1,2] as well as for COS hydrolysis to CO2 and for the Claus process [3±5]. They are also constituents of the catalysts for the oxidative dehydrogenation of butene to butadiene [6] and are active in the selective catalytic reduction of NOx by ammonia [7]. The addition of Al oxide improves the morphological stability and the attrition resistance of catalysts [8]. In previous papers, we reported the results of our investigations on coprecipitated binary Fe±Cr [9,10], Fe±Al [11,12] and Cr±Al [13] oxide catalytic materials. In agreement with the thermodynamics of these systems we obtained corundum-type solid solutions in the entire compositional range for the system Fe±Cr, while for Fe±Al and Cr±Al systems spinel-type solid solutions have been obtained at low temperature, converting into mixtures of two corundumtype solid solutions or triphasic systems with both spineltype and corundum-type phases. Metastable corundum-type solid solutions have also been obtained with compositional ranges out of those corresponding to thermodynamic solubilities in both cases. For Fe±Cr±Al oxide ternary system, according to thermodynamic data, a single corundum-type solid solutions with *Corresponding author. every Al content can be obtained only at T  1773 K and only with Fe : Cr atomic ratio lower than 1 : 1, that is, Fe content minor than Cr one [14]. By lowering the equilibrium temperature the miscibility gap extends to even higher Fe : Cr atomic ratios. By further lowering the equilibrium temperature, also iron-free Cr±Al oxide binary systems present a miscibility gap, but the exact temperature limit is under debate [7,15] In this paper we will summarise the results of our studies on the preparation of Fe±Cr±Al oxide ternary systems with Fe : Cr, 1 : 1 atomic ratio, as an effect of Al content and heating temperature. 2. Experimental Samples with compositions denoted as (Fe,Cr)2-x AlxO3 (with x ˆ 0, 0.20, 0.4, 0.7, 1, 1.6, 2. Fe and Cr nominal atomic ratios always ˆ 1) have been prepared by coprecipitation, by mixing three aqueous solutions (concentration 0.05±0.1 M) containing the required amounts of salts precursors Fe(NO3)2
9H2O , Cr(NO3)3
9H2O and Al(NO3)3
9H2O at pH 7.5 by addition of NH4 HCO3; then stirring continuously for 24 h at 343 K, ®ltering and drying the gel at 393 K for several hours. Nitrates and residual organic compounds have been decomposed, in air, at 673 K for 3 h and ®nally some portion of each sample was calcined 0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 0 5 6 - 5 169 J.M.G. Amores et al. / Materials Chemistry and Physics 60 (1999) 168±176 in air at 1173 K for 3 h. The elemental chemical analyses have been carried out using a Plasma II Perkin±Elmer emission spectrometer after dissolution of the samples in an HF-HNO3 mixture. The XRD spectra have been recorded on a Siemens D500 diffractometer (Cu Ka radiation, Ni ®lter; 35 kV, 35 mA). Cell parameters have been calculated by a dedicated least square software. The FT±IR spectra have been recorded using a Nicolet Magna 750 Fourier Transform instrument. For the region 4000±400 cmÿ1 a KBr beam splitter has been used with a DTGS detector. For the FIR region (600±50 cmÿ1) a `solid substrate' beam splitter and a DTGS polyethylene detector have been used. KBr pressed disks (IR region) or polyethylene pressed disks (FIR region) were used. The UV±Vis spectra analysis have been performed with a JASCO V-570 spectrophotometer in the region 200± 2500 nm using pressed disks of the samples and a polymer as a reference. The BET surface areas have been measured with a conventional volumetric instrument by nitrogen adsorption at 77 K. DTA-TG experiments were performed in air, with a Setaram TGA 92-12 apparatus, from room temperature to 1273 K, with heating and cooling rate of 10 K/min. 3. Results and discussions 3.1. Chemical analysis and surface areas The elemental chemical compositions are listed in Table 1, as measured by chemical analysis of the mixed oxides after calcination at 1173 K, together with the nominal compositions and the BET surface areas measured for samples calcined at 673 and 1173 K. The experimental compositions correspond well with the nominal ones due to a good pH control, allowing almost a complete precipitation of Al hydroxide in spite of the well-known amphoteric character. The speci®c surface areas after decomposition at 673 K show a minimum for the sample with x ˆ 0.7 and tend to increase both for lower and higher contents of aluminium. However, after calcination at 1173 K the behaviour is different. All samples decrease their surface areas, as usual upon increasing calcination temperature, except for the (Fe,Cr)1.3 Al0.7O3 and (Fe,Cr)0.4 Al1.6O3 samples whose surface area increases. This tendency for the (Fe,Cr)0.4 Al1.6O3 samples can be explained bearing in mind that in this samples chromate and carbonate species (see below) are present on the surface and decompose giving rise to weight losses, as observed in TG curves, which depend on chromate amount. According to previous studies [16±18], the amount of chromates depends strongly on surface area and the absorption bands associated with chromate species found in UV±Vis electronic spectrum are particularly intense. 3.2. Solid-state characterisation Samples dried at 393 K and calcined at 673 K. 3.3. XRD studies The XRD pattern of the Al-free precipitate dried at 393 K fully agrees with that of the solid solution a-(Fe,Cr)OOH (goethite-type, ICDD, ®le n8 29-713), as described elsewhere [9]. The XRD patterns of the mixed samples show that they are almost completely amorphous with small traces of crystalline phases. In the samples with 0 < x < 0.7 (low Al content) the crystalline phases ammonium iron carbonate hydroxide hydrate (ICDD, ®le n8 22-1039), ammonium aluminium carbonate hydroxide hydrate (ICDD, ®le n8 29-106) and ammonium aluminium chromium carbonate hydroxide hydrate [13] are present. By increasing aluminium content, a mixture of aluminium hydroxides, mainly boehmite, and `iron chromium hydroxide' Fe(CrO4)OH (ICDD, ®le n8 20-0511) is obtained. The pure aluminium precipitate is g-AlOOH (boehmite, ICDD n8 21-1307). The XRD patterns of decomposition products at 673 K are compared in Fig. 1. In the sample with x ˆ 0 a well crystallised phase a-FeCrO3 [7] is observed. By increasing the Al content, this phase almost disappears. The material is almost completely amorphous although a poorly crystalline Table 1 Chemical compositions and surface areas of (Fe,Cr)2-x AlxO3 samples Nominal composition Samples Surface area (m2/g) Experimental composition (Fe,Cr)/Al nominal Al Cr Fe (Fe,Cr)/Al experimental Annealing temperature (K) 673 a. FeCr O3 b. (Fe,Cr)1.8 Al0.2O3 c. (Fe,Cr)1.6 Al0.4O3 d. (Fe,Cr)1.3 Al0.7O3 e. (Fe,Cr)1 Al1O3 f. (Fe,Cr)0.4 Al1.6O3 g. Al2O3 1 9 4 2 1 0.25 0 0 0.20 0.40 0.68 1.00 1.60 2 0.90 0.92 0.82 0.68 0.50 0.20 0 1.10 0.88 0.78 0.64 0.50 0.20 0 1 8.8 3.9 1.9 1.0 0.3 0 125 60 43 5 16 23 228 1173 7 4 7 8 8 57 102 170 J.M.G. Amores et al. / Materials Chemistry and Physics 60 (1999) 168±176 Fig. 1. XRD patterns of the powders after calcination at 673 K. (a) FeCrO3, (b) (Fe,Cr)1.8 Al0.2O3, (c) (Fe,Cr)1.6 Al0.4O3, (d) (Fe,Cr)1.3 Al0.7O3, (e) (Fe,Cr)1Al1O3, (f) (Fe,Cr)0.4 Al1.6O3, (g) Al2O3. mixed defective spinel g-(Fe,Cr,Al)2O3 could be present. g-Al2O3 is found in the sample with x ˆ 2. Thus, it can be deduced Al tends to hinder crystallisation and the g ! a transition phase of a-FeCrO3. This is con®rmed by the samples calcined at 773 K (Fig. 2) whose XRD patterns are those of well-crystallised a-(Fe,Cr,Al)2O3 when Al content is low (Fig. 2(a,b,c)), but sharply losses crystallinity by increasing the Al content. Fig. 3. FT±IR spectra of the powders after calcination at 673 K in the 1000±400 cmÿ1 region. (a) FeCrO3, (b) (Fe,Cr)1.8 Al0.2O3, (c) (Fe,Cr)1.6 Al0.4O3, (d) (Fe,Cr)1.3 Al0.7O3, (e) (Fe,Cr)1 Al1O3, (f) (Fe,Cr)0.4 Al1.6O3, (g) Al2O3. 3.4. FT±IR and DR UV±Vis studies The FT±IR spectra of the samples at 673 K in region 1000±400 cm-1 are compared in Fig. 3. The FeCrO3 sample (Fig. 3(a)) is characterised by two strong absorption bands near 555 and 610 cmÿ1 and other weaker ones at 410, 460 and 490 cmÿ1, all of them associated with localised vibrations, namely those of FeO6 and CrO6 octahedra [19]. Finally, a net absorption band at 800 cmÿ1 and another near 1400 cmÿ1 are found probably due to residual carbonates still present. When Al is present in the samples (Fig. 3(b,c,d,e)), new strong absorptions in the region 1200±600 cmÿ1 appear, overlapping to those of FeCrO3 Fig. 2. XRD patterns of the powders after calcination at 773 K. (a) (Fe,Cr)1.8 Al0.2O3, (b) (Fe,Cr)1.6 Al0.4O3, (c) (Fe,Cr)1.3 Al0.7O3, (d) (Fe,Cr)1 Al1O3, (e) (Fe,Cr)0.4 Al1.6O3. sample until they become indistinguishable from each other. These features have been associated with localised vibrations of AlO4 tetrahedra in g-Al2O3 structure [20±22] in good agreement with the XRD analyses. Additionally, a new absorption band is observed at 950 cmÿ1 increasing in intensity by increasing the Al content. This band can be assigned to surface chromate species whose amount increases by increasing n the speci®c surface area of the sample. In Fig. 3(f,g), the typical absorptions of g-Al2O3 are predominant. The UV±Vis spectra of (Fe,Cr)2-x AlxO3 samples are showed in Fig. 4. Two clear effects are observed when alumina is dissolved into the FeCrO3 phase: (i) absolute absorption decreases notably with respect to that of FeCrO3 sample, (ii) region above 400 nm starts being a broad absorption and then transforms into an absorption tail. The FeCrO3 spectrum (Fig. 4(a)) is quite similar to those described by us in previous papers for spinel structures [9,23]. In the region below 400 nm two absorption bands at 270 (shoulder) and 370 nm are observed. These bands are due to O2ÿ ! Cr6‡ charge transfer transition of chromate species [24±26] that, in this case, is superimposed to O2ÿ ! Fe3‡ charge transfer transition of octahedral Fe3‡. Otherwise, at least three absorption shoulders are found near 500, 550 and 710 nm, and additional absorption starting near 800 nm following into the near IR region with a maximum at 1380 nm (see Fig. 4). These features are J.M.G. Amores et al. / Materials Chemistry and Physics 60 (1999) 168±176 171 band at 358 nm split into two weak bands at 351 and 382 nm, and other new features appear at 274 (shoulder), 417 and 443 nm. This can be interpreted as a consequence of the decrease in intensity, giving rise to better resolved bands, attributed to speci®c electronic transitions. The shoulder near 274 nm is associated with another O2ÿ ! Cr6‡ charge transfer of surface chromate species [32] and the absorptions at 417 and 443 nm are reasonably assigned to 4 A2g F† !4 T1g F† d ! d transition of octahedral Cr3‡ and 6 A1 !4 T2 4 D† d ! d transition of tetrahedral Fe3‡ [33]. In the region above 400 nm where aluminium oxide species do not present absorption bands (Fig. 4(g)), new features become distinguishable as shoulders near 478, 500, 561, 681, 710 and 858 nm. Part of them has been assigned to FeCrO3 electronic transitions. Meanwhile, those at 478 and 858 nm are related to additional 4 A2g F† !4 T1g F† spinallowed d ! d transition of octahedral Cr3‡ and 6 A1 F† ! 4 T1 4 G† d ! d transition of octahedral Fe3‡ [9,13,34]. Finally, the absorption band at 681 nm has a more complex interpretation, since it is unusually strong in the spectrum compared to those of a-Fe2O3 [9,35] and FeCrO3. The position could agree with 6 A1 F† !4 T1 4 G† d! d transition of tetrahedral Fe3‡ ions, that would occupy positions in the spinel-type structure discussed above. These data agree with XRD and FT±IR analyses showing poorly crystalline defective spinel-type (Fe,Cr,Al)2O3 solid solutions. Samples calcined at 1173 K. 3.5. XRD studies Fig. 4. UV±Vis spectra of the powders after calcination at 773 K. (a) FeCrO3, (b) (Fe,Cr)1.8 Al0.2O3, (c) (Fe,Cr)1.6 Al0.4O3, (d) (Fe,Cr)1.3 Al0.7O3, (e) (Fe,Cr)1 Al1O3, (f) (Fe,Cr)0.4 Al1.6O3, (g) Al2O3. associated with 4 A2g F† !2 T2g F† d ! d transition of octahedral Cr3‡ ; Fe3‡ ! Fe3‡ intercationic charge transfer transition, 4 A2g !2 T1g (F) crystal-®eld d ! d transition of octahedral Cr3‡ [11,27] and 5 E !5 T2 d ! d transition of both octahedral Fe3‡ and Cr3‡ (its major energy with respect to that of crystal ®eld
o is due to a distortion of octahedral coordination [28±30]), respectively. The Al2O3 spectrum (Fig. 4(g)) presents quite weak absorption bands at 220 and 300 nm that are likely due to cation impurities in the precursor. The spectra of the samples with a low Al content (Fig. 4(b,c)) have notably changed as a clear effect of alumina dissolution into FeCrO3 ceramic matrix. A broad absorption band now appears in the region below 400 nm. Moreover, weak absorption maxima at 262 and 356 nm are observed, related to O2ÿ ! Cr6‡ charge transfer transition as discussed above, although they are shifted towards lower wavelengths with respect to pure FeCrO3 spectrum. This changes can be associated with alumina dissolution into the structure, which changes the ionicity of Me±O bond, as hypothesised by Kadenatsi et al. for other mixed oxides [31]. For higher Al contents (Fig. 4(d,e,f)), the absorption In Fig. 5, the XRD patterns of the samples calcined at 1173 K are shown. From the samples with x ˆ 0 to x ˆ 0.7, the pattern of the a-(Fe,Cr)2O3 phase (hereinafter denoted as a1) is substantially found with decreasing intensity by increasing the Al content and shifting the peaks to lower d-spacings. Additionally, g/q-Al2O3 and traces of a-Al2O3 (hereinafter denoted as a2) are observed for higher Al Fig. 5. XRD patterns of the powders after calcination at 1173 K. (a) FeCrO3, (b) (Fe,Cr)1.8 Al0.2O3, (c) (Fe,Cr)1.6 Al0.4O3, (d) (Fe,Cr)1.3 Al0.7O3, (e) (Fe,Cr)1 Al1O3, (f) (Fe,Cr)0.4 Al1.6O3, (g) Al2O3 (a1: *, a2: ‡, q-g-Al2O3: x, a-Al2O3: o). 172 J.M.G. Amores et al. / Materials Chemistry and Physics 60 (1999) 168±176 contents, while the peaks of a1 are going to disappear. The pure Al sample is characterised by the presence of poorly crystalline transitional aluminas with small peaks of the corundum phase. The cell parameters calculated of the a1 and a2 phases are summarised in Table 2. As for the a1 phase, a continuous contraction of cell parameters and volume is observed with increasing Al content. In agreement with Vegard's law [36], this indicates that alumina is increasingly dissolved into the FeCrO3 a1 phase forming a solid solution. The solubility of alumina into a-FeCrO3 calculated with the Vegard's law, 15%, is only a little bit lower than the nominal one for samples with x  0.4, that are monophasic. In biphasic samples solubilities above 30% are measured. Conversely, the cell parameters and volume of the a2 phase, apparently tends to grow up. This is understood considering the presence of a third spinel-type phase and the catalytic effect of Fe3‡ and Cr3‡ ions favouring the q-Al2O3 ! a-Al2O3 (a2) phase transition. 20% of FeCrO3 is soluble in a-Al2O3, according to our measures based on the Vegard's law. In parallel, a progressive g ! q ! a phase transformation of Al2O3 is produced. 3.6. FT±IR and DR UV±Vis studies The FT±IR spectra of samples calcined at 1173 K are shown in Fig. 6(a,b,c,d,e,g). A spectrum of well-crystallised a-Al2O3 has been added (Fig. 6(h)) for comparison. In general, the mixed oxides spectra have signi®cantly changed in the 1000±400 cmÿ1 region, giving rise to wellde®ned bands typical of crystalline materials, as just seen in XRD analyses. These features are also observed in the FeCrO3 sample, where the carbonate band at 882 cmÿ1 have already disappeared. On the contrary, the sample of pure Al2O3 presents broad bands analogous to those of the sample calcined at 673 K. Additionally, some weak typical bands of a-Al2O3 near 402 and 692 cmÿ1 are found (Fig. 6(g,h)). At quite low Al content, (Fig. 6(b)) the absorption bands found in FeCrO3 sample at 301, 381, 408, 588, 660 and 526 cmÿ1 [9] shift towards higher wavenumbers. This behaviour has already been reported in solid solutions and is due to essentially localised vibrations of MeO6 octahedra (Me ˆ Fe3‡, Cr3‡, Al3‡) [19]. The band near 618 cmÿ1 assignable to AlO6 tetrahedra in a corundum-type structure and the broad bands above 670 cmÿ1, assignable to transitional aluminas, progressively grow (Fig. 6(c,d,e,f)) [22]. This merging picture agrees with the formation of a1 solid solution and later with the formation of transitional aluminas [9]. Finally, we remark again the appearence of bands in 900±700 cmÿ1 region in agreement with XRD. In Fig. 7, UV±Vis spectra of samples after calcination at 1173 K are compared. All of them have notably been transformed as a direct effect of increasing calcination temperature. The spectrum of FeCrO3 presents features analogous to those discussed for the spectrum of the sample calcined at 673 K at 260, 380, 470 and 704 nm. Instead, the Fig. 6. FT±IR spectra of the powders after calcination at 1173 K in the 1000±50 cmÿ1 region. (a) FeCrO3, (b) (Fe,Cr)1.8 Al0.2O3, (c) (Fe,Cr)1.6 Al0.4O3, (d) (Fe,Cr)1.3 Al0.7O3, (e) (Fe,Cr)1 Al1O3, (f) (Fe,Cr)0.4 Al1.6O3, (g) Al2O3, (h) a- Al2O3. broad absorption in the near IR region as well as that at 858 nm assigned to tetrahedral Fe3‡ disappear. Otherwise, when a bit of Al is added, the bands of octahedral Cr3‡ increase in intensity with respect to those assigned to octahedral Fe3‡ (Fig. 7). At higher Al contents, the spectra are progressively dominated by typical O2ÿ ! Cr6‡ charge transfer transitions of surface chromate species at 263 and 360 nm [32], while chromium and iron absorption maxima are weaker and weaker [34] (Fig. 7(c,d,e,f)). This is clearly due to the dilution effect of Al into FeCrO3 matrix, as discussed above. 3.7. Thermal analyses studies The DTA curves in 673±1273 K range and TG weight losses of samples after previous calcination at 673 K are shown in Fig. 8 and Table 3, respectively. The Al-free sample (Fig. 8(a)) does not present signi®cant thermal features, because the a-FeCrO3 phase has already crystallised and is thermodynamically stable. In this range a total weight loss of 2% is detected in TG experiments attributed to decomposition of surface chromate and carbonate species. Samples a1 (a-FeCrO3) Ê) a (A a2(a-Al2O3) Ê) b (A Ê3 Ê) c (A V (A ) a a-Fe2O3 a-Cr2O3 a. FeCr O3 673 K 1173 K b. (Fe, Cr)1.8 Al0.2O3 c. (Fe, Cr)1.6 Al0.4O3 d. (Fe, Cr)1.3 Al0.7O3 e. (Fe, Cr)1 Al1O3 f. (Fe,Cr)0.4 Al1.6O3 g. Al2O3 a-Al2O3 5.030 4.960 4.990 4.997 4.977 4.961 4.951 4.927 4.906 (1) (3) (2) (2) (1) (1) (1) (2) (5) 5.030 4.960 4.990 4.997 4.977 4.961 4.951 4.927 4.906 (1) (3) (2) (2) (1) (1) (1) (2) (5) tw: this work and, ICDD file 42-1468 (synthetic corundum). 13.730 13.560 13.620 13.619 13.541 13.547 13.482 13.471 13.313 (3) (7) (7) (10) (9) (6) (5) (8) (13) 301 289 294 295 291 289 286 283 277 0 0 Ê) a (A % Al2O3 in a1 Ê) b (A Ê) c (A Ê 3) V (A c % FeCrO3 in a2 a Refã c 7 7 9 15 20 29 38 12 12 22 25 40 4.789 (3) 4.799 (2) 4.803 (2) 4.740 (2) 4.7588 (1) 4.789 (3) 4.799 (2) 4.803 (2) 4.740 (2) 4.7588 (1) 13.110 13.100 13.120 13.010 12.992 (2) (9) (7) (1) (1) 260 261 262 253 255 13 17 19 18 17 20 tw tw tw tw tw tw tw ICDD J.M.G. Amores et al. / Materials Chemistry and Physics 60 (1999) 168±176 Table 2 Cell parameter of (Fe,Cr)2ÿx AlxO3 samples 173 174 J.M.G. Amores et al. / Materials Chemistry and Physics 60 (1999) 168±176 Table 3 Losses of weight in TG experiments Fig. 7. UV±VIS spectra of the powders after calcination at 1173 K. (a) FeCrO3, (b) (Fe,Cr)1.8 Al0.2O3, (c) (Fe,Cr)1.6 Al0.4O3, (d) (Fe,Cr)1.3 Al0.7O3, (e) (Fe,Cr)1 Al1O3, (f) (Fe,Cr)0.4 Al1.6O3, (g) Al2O3. Samples Temperature (K) Weight loss (%) FeCrO3 380±650 650±970 0.9 1.0 (Fe,Cr)1.8 Al0.2O3 430±534 534±970 2.5 2.0 (Fe,Cr)1.6 Al0.4O3 420±600 600±970 5.1 2.8 (Fe,Cr)1.3 Al0.7O3 420±711 771±970 6.1 3.2 (Fe,Cr)1 Al1O3 428±711 711±970 4.1 3.4 (Fe,Cr)0.4 Al1.6O3 410±780 780±970 5.7 4.5 Al2O3 310±970 10 For the mixed oxide samples (Fig. 8(b,c,d,e,f)), a net exothermic peak is observed shifting towards higher temperatures by increasing Al content with additional previous exothermic shoulder. These features are due to the spineltype ! corundum-type (q ! a) phase transition, preceded by particle sintering. The shift of phase transition temperature can be explained by a hindering effect by aluminum in agreement with XRD analyses. Such phase transition results in the segregation of a1 and a2 depending on the nominal composition of each sample. Phase transition is not detected at high Al content, because it well occurs above 1273 K (Fig. 8(f)). Moreover, increasing extends of weight losses are found in two steps by increasing Al content (Table 3). The second weight loss step is related to the g ! a phase transition (occurring with loss of surface area) and to the decomposition of surface chromate species, as discussed above. All samples after DTA-TG runs up to 1273 K were analysed by XRD. The patterns did not present signi®cant differences with respect to those carried out at 1173 K. Only slight changes in cell parameters of a1 and a2 phases were found, in agreement with the equilibrium between these two solid solutions. Moreover, the amount of q-Al2O3 phase is strongly lowered as a consequence of increasing calcination temperature. These data agree with those reported in FT±IR, UV±Vis and XRD analyses, which indicate evidence of CrO42ÿ species in surface for all mixed oxides, but mainly when samples have relative high speci®c surface area. 4. Conclusions The main conclusions from this work are the following: Fig. 8. DTA curves of coprecipitated samples after calcination at 673 K. (a) FeCrO3, (b) (Fe,Cr)1.8 Al0.2O3, (c) (Fe,Cr)1.6 Al0.4O3, (d) (Fe,Cr)1.3 Al0.7O3, (e) (Fe,Cr)1 Al1O3, (f) (Fe,Cr)0.4 Al1.6O3, (g) Al2O3. 1. (Fe,Cr)2ÿx AlxO3 samples have been prepared by coprecipitation and calcination at 673 and 1173 K. J.M.G. Amores et al. / Materials Chemistry and Physics 60 (1999) 168±176 175 Fig. 9. Aproximate metastable phase diagram for Fe2O3±Cr2O3±Al2O3 system prepared by coprecipitation and subsequent calcined at 673 K (left) and at 1173 K (right). (a1, a2 and a3 are corundum-type solid solutions; Sp is spinel-type solid solution and Am is amorphous). 2. The Al-free sample is constituted by the corundum-type phase a-FeCrO3, while Al addition results in the formation of poorly crystallised spinel-type g-(Fe,Cr,Al)2O3 after calcination at 673 K. 3. Calcination at 1173 K results in the formation of one corundum-type solid solution phase (a1) only for x  0.7. With high Al contents, segregation of two corundum-type solid solutions a1 and a2 and of a spinel-type phase is found. 4. Al strongly hinders the spinel-type ! corundum-type phase transition in Fe±Cr oxides. 5. The trend of the specific surface areas shows a minimum at x ˆ 0.7 for the samples calcined at 673 K. For samples calcined at 1173 K the specific surface areas are almost unchanged with Al contents up to x ˆ 1 and increase strongly with increasing the amount of the theta-spineltype phase for x > 1. 6. UV, IR and TG data show that, the higher the specific surface areas, the higher the amounts of surface species involving hexavelant chromium, chromate species, in the ternary samples. Our investigations on catalytic materials belonging to the systems of the sesquioxides of the trivalent elements Al, Fe and Cr include the works published previously concerning Fe±Cr mixed oxides [9,10], Al±Fe mixed oxides [11,12] and Al±Cr mixed oxides [13] and few additional unpublished experiments. As a general conclusion, we can propose the following metastable phase diagrams for the system (Fig. 9). These data refer to materials that we prepared using the same coprecipitation technique followed by calcination at 673 K (left) and 1273 K (right). Although these proposed `phase diagrams' are certainly very approximate, we believe that they can be useful for the preparation and design of Al±Cr±Fe mixed oxide catalysts by similar techniques. 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